Method for manufacturing thin film compound solar cell

ABSTRACT

To manufacture a thin film compound solar cell which can improve the adhesive property of electrodes even when being provided with a base material, and which prevents the base material from being separated. A cell main body configured by laminating a plurality of compound semiconductor layers is formed on a substrate. A rear surface electrode  7  is formed on the cell main body, and a rear surface film  8  as the base material is formed on the rear surface electrode  7 . A reinforcing material  9  is attached on the rear surface film  8 . The substrate is separated from the cell main body, and the cell main body is mesa-etched. A surface electrode  13  is formed on a contact layer  3  after the etching. The reinforcing material  9  is separated, and the surface electrode  13  is annealed. The formed thin film compound solar cell is separated into a plurality of solar cell elements.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a thin filmcompound solar cell having a cell main body in which at least one PNjunction is formed by a plurality of compound semiconductor layers, eachhaving a different chemical composition.

BACKGROUND ART

A conventional thin film compound solar cell is configured such that asurface electrode is provided on a light receiving surface of a cellmain body formed by laminating a plurality of compound semiconductorlayers, and such that a rear surface electrode is provided on thesurface opposite to the light receiving surface of the cell main body.

The thin film compound solar cell is manufactured as follows. In processA1 shown in FIG. 24, an etching-stop layer 102, a base layer 103, anemitter layer 104, and a contact layer 105 respectively formed bycompound semiconductor layers are laminated in this order on a substrate101, so that a cell main body is formed.

In process A2 shown in FIG. 25, a protective film, such as aphotoresist, is applied on the surface of the contact layer 105, and theregion of the protective film, which region is patterned by exposure, isetched. The contact layer 105 is patterned by contact layer etching. Theapplied resist is removed after the completion of the patterning. Next,a photoresist is again applied for formation of a surface electrode, anda protective film is formed.

In process A3 shown in FIG. 26, a protective film opening section isformed by patterning the photoresist by exposure so that the protectivefilm opening section is included in the region of the contact layer 105formed by the preceding process. After a surface electrode is laminated,the photoresist is removed, so that a surface electrode 106 isselectively formed only in the protective film opening section. By thissurface electrode forming process, the region of the surface electrodecan be patterned so as to be included in the region of the contact layerformed by the preceding process.

After the completion of the patterning of the surface electrode 106, thesurface electrode 106 is annealed at a temperature of about 350° C. inorder to reduce the component of contact resistance between the surfaceelectrode 106 and the contact layer 105 and to increase the adhesiveforce between the surface electrode 106 and the contact layer 105.

In process A4 shown in FIG. 27, the protective film is patterned byexposure so as to define a cell formation region corresponding to apredetermined shape (chip shape) of a solar cell element. A protectivefilm opening section is formed, so that the opening section ismesa-etched. Then, the solar cell element having the predetermined shape(chip shape) is separated by mechanical means, such as dicing.

In process A5 shown in FIG. 28, a transparent resin, such as siliconeresin, is applied to the side of the light receiving surface of thesolar cell element, so that a transparent surface film 107 is bondedonto the transparent resin. Thereby, the thin film compound solar celland the surface film 107 are bonded to each other via the resin, so thatthe surface film 107 serves as a base material of the thin film compoundsolar cell.

In process AG shown in FIG. 29, a reinforcing material 108, such asglass or sapphire, is bonded via wax on the side of the light receivingsurface of the solar cell element, to which side the surface film 107 isbonded.

In process A7 shown in FIG. 30, the solar cell element, to which thereinforcing material 108 is bonded, is immersed in an etchant. Since theetching is stopped at the etching-stop layer 102, only the substrate 101can be removed so that only the cell main body is left. Thereby, thesubstrate 101 is separated from the compound semiconductor layers, sothat the solar cell element exhibits its flexibility.

In process A8 shown in FIG. 31, an electrode material is vapor-depositedon the exposed rear surface of the compound semiconductor layer, so thata rear surface electrode 109 is formed.

In process A9 shown in FIG. 32, the wax bonding the reinforcing material108 to the solar cell element is finally is dissolved by an organicsolvent, such as acetone, so that the reinforcing material 108 isremoved from the solar cell element.

The thin film compound solar cell manufactured as described above hasthe structure in which the surface film as the base material is bondedto the light receiving surface of the cell main body with a PN junctionformed therein.

Meanwhile, the surface film is bonded on the side of the light receivingsurface, and hence high transparency is required for the surface film soas to prevent the conversion efficiency of the solar cell element frombeing impaired. The high transparency film generally has low temperatureresistance. In the conventional method for manufacturing the thin filmcompound solar cell, the process is performed such that, after thesurface film is bonded to the solar cell element, the substrate isremoved and the rear surface electrode is formed. After the formation ofthe rear surface electrode, it is necessary to anneal the rear surfaceelectrode in order to reduce the component of contact resistance betweenthe rear surface electrode and the compound semiconductor layer, and inorder to increase the adhesive force between the rear surface electrodeand the compound semiconductor layer. The annealing temperature ishigher than the heat-resistant temperature of the surface film, andhence the rear surface electrode cannot be annealed in the state wherethe surface film is bonded to the solar cell element. Therefore, thereis a problem that the rear surface electrode is separated from thecompound semiconductor layer.

Further, when the wax used to bond the solar cell element to thereinforcing material is dissolved by the organic solvent, the resin usedto bond the compound semiconductor layer to the surface film is alsoexposed to the organic solvent at the same time. Thus, there is aproblem that, when the resin is exposed to the organic solvent andwater, the exposed resin penetrates into the interface between thesurface film and the resin, or into the interface between the compoundsemiconductor layer and the resin, so as to make the surface film liableto be separated from the compound semiconductor layer.

Further, the process for removing the substrate by the etchant isperformed after a metallic ribbon for establishing electrical connectionis welded to the solar cell element. As the etchant for etching thesubstrate, it is necessary to use hydrofluoric acid, and the like,depending on the substrate material. However, hydrofluoric acid reactswith the metallic ribbon to corrode the metallic ribbon. At the time ofetching the substrate, the exposed metallic ribbon is required to beprotected by being covered with an acid resistant material. This resultsin a problem that the number of processes is increased.

Here, a method for manufacturing the compound solar cell described inPatent Document 1 includes: forming the rear surface electrode on thecell main body; attaching a support plate on the rear surface electrode;separating the substrate from the cell main body to expose the surfaceof the cell main body; forming the surface electrode on the exposedsurface of the cell main body; and then removing the support plate.

In the compound solar cell described in Patent Document 1, the rearsurface electrode is formed first. For this reason, after the rearsurface electrode is annealed, the surface film can be bonded. However,in the above-described compound solar cell described in Patent Document,the surface film as the base material is not provided, and hence theproblem of separation of the surface film does not occur.

PRIOR ART DOCUMENTS Patent Document

-   Patent Document 1: Japanese Patent Laid-Open No. 2004

SUMMARY OF INVENTION Technical Problem

The structure of the solar-battery cell described in Patent Document 1has a problem that, since the structure is configured only by thesemiconductor epitaxial layer and the rear surface electrode, theepitaxial layer is easily broken when an external force due to bending,or the like, is applied to the structure. Further, in the structureconfigured only by the semiconductor epitaxial layer and the rearsurface electrode, the warpage of the solar-battery cell cannot becontrolled.

Thus, in view of the above, an object of the present invention is toprovide a method for manufacturing a thin film compound solar cell whichcan improve the adhesive property of the electrode even when providedwith the base material and which can withstand the external forcewithout the separation of the base material.

Means for Solving the Problems

The present invention provides a method for manufacturing a thin filmcompound solar cell having a cell main body in which at least one PNjunction is formed by a plurality of compound semiconductor layers, eachhaving a different chemical composition, the method including: a processof forming a cell main body by forming an etching-stop layer forsuppressing infiltration of an etching solution from the side of asubstrate, a contact layer, an emitter layer made of a firstconductivity type compound semiconductor, a base layer forming a PNjunction with the emitter layer, and a buffer layer; a process offorming a rear surface electrode on the cell main body; a process ofannealing the rear surface electrode; a process of forming a basematerial on the rear surface electrode; a process of attaching areinforcing material on the base material; a process of separating thesubstrate from the cell main body; a process of forming a surfaceelectrode on the exposed surface of the separated cell main body; aprocess of separating the reinforcing material; and a process ofsintering the surface electrode. Further, the method for manufacturingthe thin film compound solar cell includes a process of, after theannealing of the surface electrode, separating the thin film compoundsolar cell into a plurality of solar cell elements, and a process ofconnecting a metallic ribbon to each of the electrodes.

The rear surface electrode is formed in the early stage, and thereby therear surface electrode can be annealed, so that it is possible toimprove the adhesive property of the electrode and to reduce the contactresistance of the electrode. Further, a metallic ribbon is finallyconnected, and thereby it is possible to eliminate the unnecessaryprotection for the metallic ribbon.

In the method for manufacturing the thin film compound solar cell, thecell main body includes an etching-stop layer and a contact layerrespectively laminated on the side of the substrate. Also, the methodfor manufacturing the thin film compound solar cell includes a processof, after the separation of the substrate, removing the etching-stoplayer from the cell main body, a process of etching the contact layerinto a predetermined pattern, and a process of mesa-etching the cellmain body, and forms the surface electrode on the mesa-etched contactlayer.

Alternatively, the method for manufacturing the thin film compound solarcell includes a process of, after the separation of the substrate,removing the etching-stop layer from the cell main body, a process ofmesa-etching the cell main body, and a process of, after the surfaceelectrode is formed on the contact layer, etching the contact layer. Inthis case, the surface electrode functions as an etching mask.

The base material is made of a material having heat resistance capableof withstanding a temperature higher than the annealing temperature ofthe surface electrode and is made of, for example, a film-likepolyimide. The polyimide film is formed by applying and annealing aresinous polyimide. Alternatively, the polyimide film is formed byapplying and annealing a solution of polyamic acid which is a polyimideprecursor. Further, the thickness of the polyimide film is set to 15 μmor less.

Here, it is technically impossible to bond the polyimide film onto therear surface electrode by using an adhesive, because of the problem ofthe heat-resistance of the adhesive itself. Thus, when the base materialis formed by the above-described method, the surface electrode can besintered after the formation of the base material.

With the above-described manufacturing method, a thin film compoundsolar cell is manufactured, which includes: a compound semiconductorlayer with at least one PN junction formed therein; a surface electrodeformed on one surface of the compound semiconductor layer; a polyimidefilm formed on the other surface of the compound semiconductor layer;and a rear surface electrode sandwiched between the compoundsemiconductor layer and the polyimide film. Note that the compoundsemiconductor layer is made of an epitaxially grown single crystal thinfilm.

Effects of the Invention

According to the present invention, since the surface electrode and therear surface electrode are annealed, it is possible to improve theadhesive property of each of the electrodes and to reduce the contactresistance of each of the electrodes. Further, the heat-resistant basematerial is used. Thus, after the reinforcing material is removed, theelectrode can be annealed in the state where the base material isattached. As a result, the reinforcing material is not heat-treated, andhence the reinforcing material can be reused.

Further, a highly heat-resistant film, such as polyimide, is used as thebase material, and hence the film itself serves as the supporting body.Therefore, even when an external force is applied, the photovoltaic cellis not broken. Further, since the amount of warpage of the photovoltaiccell is changed according to the film thickness, it is possible tocontrol the warpage of the cell during the film forming process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a thin film compound solar cellaccording to the present invention when a cell main body made of aplurality of compound semiconductor layers is formed.

FIG. 2 is a cross-sectional view of the thin film compound solar cellwhen a rear surface electrode is formed.

FIG. 3 is a cross-sectional view of the thin film compound solar cellwhen a rear surface film is formed.

FIG. 4 is a cross-sectional view of the thin film compound solar cellwhen a reinforcing material is attached.

FIG. 5 is a cross-sectional view of the thin film compound solar cellwhen a substrate is removed.

FIG. 6 is a cross-sectional view of the thin film compound solar cellwhen an etching-stop layer is removed.

FIG. 7 is a cross-sectional view of the thin film compound solar cellwhen a first protective film is formed.

FIG. 8 is a cross-sectional view of the thin film compound solar cellwhen the protective film is patterned.

FIG. 9 is a cross-sectional view of the thin film compound solar cellwhen a contact layer is etched.

FIG. 10 is a cross-sectional view of the thin film compound solar cellwhen the protective film is separated.

FIG. 11 is a cross-sectional view of the thin film compound solar cellwhen a second protective film is formed.

FIG. 12 is a cross-sectional view of the thin film compound solar cellwhen the protective film is patterned.

FIG. 13 is a cross-sectional view of the thin film compound solar cellat the time of mesa-etching.

FIG. 14 is a cross-sectional view of the thin film compound solar cellwhen the protective film is separated.

FIG. 15 is a cross-sectional view of the thin film compound solar cellwhen a third protective film formed.

FIG. 16 is a cross-sectional view of the thin film compound solar cellat the time of patterning for forming a surface electrode.

FIG. 17 is a cross-sectional view of the thin film compound solar cellwhen the surface electrode is formed.

FIG. 18 is a cross-sectional view of the thin film compound solar cellwhen the protective film is removed.

FIG. 19 is a cross-sectional view of the thin film compound solar cellwhen the reinforcing material is separated.

FIG. 20 is a cross-sectional view of the thin film compoundsemiconductor solar cell when the cell is divided into solar cellelements.

FIG. 21 is a cross-sectional view of a thin film compound solar cellwhen mesa-etching is performed before the contact layer etching inanother manufacturing method.

FIG. 22 is a cross-sectional view of the thin film compound solar cellwhen the surface electrode is formed in another manufacturing method.

FIG. 23 is a cross-sectional view of the thin film compound solar cellwhen the contact layer is etched in another manufacturing method.

FIG. 24 is a cross-sectional view of the thin film compound solar cellwhen a cell main body is formed in a conventional manufacturing method.

FIG. 25 is a cross-sectional view of the thin film compound solar cellwhen a contact layer is etched.

FIG. 26 is a cross-sectional view of the thin film compound solar cellwhen a surface electrode is formed.

FIG. 27 is a cross-sectional view of the thin film compound solar cellwhen the cell is separated into solar cell elements.

FIG. 28 is a cross-sectional view of the thin film compound solar cellwhen a surface film is formed.

FIG. 29 is a cross-sectional view of the thin film compound solar cellwhen a reinforcing material is attached.

FIG. 30 is a cross-sectional view of the thin film compound solar cellwhen a substrate is removed.

FIG. 31 is a cross-sectional view of the thin film compound solar cellwhen a rear surface electrode is formed.

FIG. 32 is a cross-sectional view of the thin film compound solar cellwhen the reinforcing material is separated.

EXPLANATION OF REFERENCES

-   -   1 substrate    -   2 etching-stop layer    -   3 contact layer    -   4 emitter layer    -   5 base layer    -   6 buffer layer    -   7 rear surface electrode    -   8 rear surface film    -   9 reinforcing material    -   10 first protective film    -   11 second protective film    -   12 third protective film    -   13 surface electrode

MODES FOR CARRYING OUT THE INVENTION

A thin film compound solar cell according to the present embodiment hasa structure including: a cell main body in which at least one PNjunction is formed by laminating a plurality of compound semiconductorlayers, each having a different chemical composition; a surfaceelectrode which is formed on the light receiving surface of the cellmain body; a rear surface electrode which is formed on the oppositesurface of the cell main body; and a base material for the thin filmsolar cell. The base material is formed on the opposite surface of thecell main body, and the rear surface electrode is sandwiched between thecell main body and the base material.

A method for manufacturing the solar cell having this structure will bedescribed. In process 1 shown in FIG. 1, a compound semiconductor layermade of single crystal thin films is formed by laminating, on asubstrate 1, an etching-stop layer 2 for suppressing infiltration of anetching solution from the side of the substrate, a contact layer 3, anemitter layer 4 made of a first compound semiconductor, a base layer 5forming a PN junction with the emitter layer 4, and a buffer layer 6 inthis order. The substrate 1 has, for example, a wafer-like form, and acell main body is formed by laminating, on the substrate 1, compoundsemiconductor layers, such as the etching-stop layer 2, the contactlayer 3, the emitter layer 4, the base layer 5, and the buffer layer 6,by a known process, for example, the epitaxial growth method describedin Patent Document 1.

As the substrate 1, it is possible to use a wafer made of a material,such as Ge, GaP, and GaAs. As the compound semiconductor layers, forexample, the etching-stop layer 2 such as an InGaP layer, the contactlayer 3 such as an AlInP layer, the emitter layer 4 such as an N-typeInGaP layer, the base layer 5 such as a P-type InGaP layer, and thebuffer layer 6 such as an AlInP layer are used.

Note that the cell main body is assumed to have the 5 layer structure,but the layer structure of the cell main body is not limited to this.The cell main body may have, for example, a four layer structure, a sixlayer structure, or the like. Further, other than the etching-stop layer2, the contact layer 3, the emitter layer 4, the base layer 5, and thebuffer layer 6, the cell main body may include compound semiconductorlayers, such as a BSF (back surface field) layer, a window layer, atunnel junction layer of a multi-junction type solar cell, and the otheremitter layer and the other base layer of the multi-junction type solarcell.

That is, it is only necessary that the cell main body formed on thesubstrate 1 is made of a plurality of compound semiconductor layers,each having a different chemical composition, and that at least one PNjunction is formed by the plurality of compound semiconductor layers.Further, it is only necessary that the plurality of compoundsemiconductor layers include at least a layer which is easily etched bya second etching solution for etching the contact layer and which ishardly etched by a third etching solution for mesa-etching, and a layerwhich is hardly etched by the second etching solution and which iseasily etched by the third etching solution. The former layer is thecontact layer 3, and the latter layers are the emitter layer 4 and thebase layer 5.

In process 2 shown in FIG. 2, a rear surface electrode 7 is formed onthe surface of the buffer layer 6 which is the outermost surface of thecompound semiconductor layers (the etching-stop layer 2, the contactlayer 3, the emitter layer 4, the base layer 5, and the buffer layer 6)laminated onto each other. The rear surface electrode 7 is formed allover the compound semiconductor layer. The rear surface electrode 7 isformed by a method in which a metal paste made of Al, Ag, or the like,is applied to the outermost surface of the cell main body by screenprinting. After the formation of the rear surface electrode 7, the cellmain body is subjected to heat treatment, so that the rear surfaceelectrode 7 is annealed. Therefore, the contact resistance between thesurface of the compound semiconductor layer and the rear surfaceelectrode 7 can be reduced, and the adhesive force between the surfaceof the compound semiconductor layer and the rear surface electrode 7 canbe increased.

In process 3 shown in FIG. 3, after the formation of the rear surfaceelectrode 7, a highly heat-resistant rear surface film S is formed onthe rear surface electrode 7. The rear surface film 8 is made of amaterial having heat resistance capable of withstanding a temperature of300° C. or more, and, for example, polyimide is used as the material ofthe rear surface film 8. The method for forming the rear surface film 8includes a method in which a varnish-like resin is applied on the rearsurface electrode 7 at a normal temperature by a spin-coating method, orthe like, and then annealed.

When the rear surface film 8 is formed by applying and annealing avarnish of polyimide, it is necessary to control the film thickness ofpolyimide. This is because, when the film thickness of polyimide is 20μm or more, air bubbles are mixed into the polyimide film, so as toprevent a flat film from being annealed, and because the amount ofwarpage of the polyimide film is also large, so as to damage the cellmain body. When the film thickness of polyimide is reduced, the mixingof air bubbles is prevented in the thickness range of 20 μm or less, andthe warpage of the polyimide film is also reduced. When the filmthickness of polyimide is about 7 μm, the amount of warpage of thepolyimide film is minimized. When the film thickness of polyimide isless than about 7 μm, the direction of warpage is reversed, and theamount of warpage is again increased. Therefore, as a result ofconsideration of the warpage amount of polyimide, and of the elasticityof polyimide as the base material of the cell main body, the filmthickness of polyimide in the range of 5 to 15 μm is suitable for theproduction of the cell main body, and in particular the film thicknessof about 7 μm is optimum. Note that, in the above, the method, in whichthe film is formed by annealing the varnish-like polyimide, isexemplified. However, besides this method, there is a method in which athermal fusion adhesive film is used and press-fitted to the rearsurface electrode 7 while being heated. Thereby, the rear surface film 8is formed as the base material of the thin film solar cell so as toserve as the supporting surface film is set to 15 μm or less, the basematerial with a less warpage amount can be formed, so that the warpageof the cell main body is controlled to reduce the warpage of the cellmain body.

In process 4 shown in FIG. 4, after the formation of the rear surfacefilm 8, a reinforcing material 9 for reinforcing the compoundsemiconductor layer is bonded onto the rear surface film 8. As thereinforcing material 9, it is preferred to use a PET film, and the like,with an adhesive material whose adhesive force is lowered by irradiationof UV light. Thereby, the reinforcing material 9 can be directlyattached to the rear surface film 8.

In process 5 shown in FIG. 5, after the bonding of the reinforcingmaterial 9, the substrate 1 is etched and removed by using a firstetching solution. As the first etching solution, a proper solution isselectively used according to the material of the substrate. In the casewhere the substrate is made of Ge, it is preferred to use a solutionhaving a composition of hydrofluoric acid:aqueous hydrogenperoxide:water=1:1:4. The etching-stop layer 2 is a layer which ishardly etched by the first etching solution. Thus, when the substrate isetched so that the etching-stop layer 2 is exposed, the progress ofetching is stopped. Thereby, only the substrate 1 can be separated sothat only the compound semiconductor layer is left.

In process 6 shown in FIG. 6, after the etching of the substrate, theetching-stop layer 2 is removed by being etched by the second etchingsolution. The contact layer 3 is exposed at the outermost surface.

In process 7 shown in FIG. 7, a first protective film 10 is applied andformed on the contact layer 3 in order to protect the outermost surfaceof the cell main body from the chemical treatment (contact layeretching). The protective film 10 has resistance against the secondetching solution used to etch the compound semiconductor layer in thesubsequent process. When a photoresist is used as the protective film10, the processing is easily and surely performed.

In process 8 shown in FIG. 8, after the formation of the protective film10, the patterning of the protective film 10 for forming a surfaceelectrode is performed by using a glass mask, so that an opening sectionis formed in the protective film 10. The protective film 10 serves as anetching mask when the contact layer is etched in the subsequent process.

In process 9 shown in FIG. 9, after the protective film 10 is patterned,the contact layer is etched. The cell main body is immersed in thesecond etching solution capable of etching the compound semiconductorlayer, so that the contact layer 3 is etched by using the patternedprotective film 10 as an etching mask. An alkali solution is used as thesecond etching solution. A part of the emitter layer 4 is exposed at theoutermost surface.

In process 10 shown in FIG. 10, after the etching of the contact layer3, the protective film 10 used as the etching mask for etching thecontact layer is separated by a lift-off method.

In process 11 shown in FIG. 11, a second protective film 11 is appliedand formed in order to protect the outermost surface of the cell mainbody from mesa-etching. A photoresist is used as the second protectivefilm 11.

In process 12 shown in FIG. 12, after the formation of the protectivefilm 11, an opening section for defining the region of a solar cellelement is formed in the protective film 11 by patterning the protectivefilm 11 by using a glass mask. The protective film 11 serves as anetching mask at the time of mesa-etching in the subsequent process.

In process 13 shown in FIG. 13, after the patterning of the protectivefilm 11, the cell main body is immersed in the third etching solutioncapable of etching the compound semiconductor layer, so that the cellmain body is mesa-etched by using the patterned protective film 11 as anetching mask. The emitter layer 4 and the base layer 5 are etched alongthe patterned protective film 11. An alkali solution and an acidsolution are used as the third etching solution. The region of the solarcell element can be defined by the mesa-etching.

In process 14 shown in FIG. 14, after the mesa-etching, the protectivefilm 11 used as the etching mask is separated by the lift-off method.

In process 15 shown in FIG. 15, a third protective film 12 made of aphotoresist is applied and formed on the whole outer surface of theetched cell main body in order to perform the patterning of the surfaceelectrode.

In process 16 shown in FIG. 16, after the formation of the protectivefilm 12, the protective film 12 is patterned by using a glass mask, andthereby an opening section is formed in the protective film 12 so thatthe patterning of the surface electrode can be performed. At this time,the patterning of the protective film 12 is performed so that theopening section is formed on the contact layer 3 patterned by thepreceding process.

In process 17 shown in FIG. 17, after the patterning of the protectivefilm 12, the cell main body, to which the reinforcing material 9 isattached, is set in an electrode forming apparatus. A surface electrode13 is formed on the protective film 12 and in the opening section of theprotective film 12. The surface electrode 13 is formed by applying anelectrode material, such as Al or Ag, to the outermost surface of thecell main body by screen printing, or by vapor-depositing the electrodematerial on the outermost surface.

In process 18 shown in FIG. 18, the cell main body, on which theelectrode material laminated, is immersed in an organic solvent, such asacetone. The photoresist, which is the protective film 12, is dissolvedin the organic solvent, so that the electrode material attached on thephotoresist is removed together with the photoresist. The electrodematerial is selectively attached only in the opening section, andthereby the surface electrode 13 is formed on the contact layer 3, sothat the thin film compound semiconductor solar cell is formed.

In process 19 shown in FIG. 19, after the formation of the surfaceelectrode 13, the thin film compound semiconductor solar cell isseparated from the reinforcing material 9. As the separation method,when the UV separation type material is used as the adhesive material, amethod is used, in which the reinforcing material 9 is separated fromthe cell main body by irradiating UV light by using a UV irradiationapparatus.

In process 20 shown in FIG. 20, after the separation of the reinforcingmaterial 9, the surface electrode 13 is annealed. By performing the heattreatment, it is possible to reduce the contact resistance between thecontact layer 3 and the surface electrode 13, and to improve theadhesive property between the contact layer 3 and the surface electrode13. Next, the thin film compound solar cell formed in the wafer isseparated into a plurality of solar cell elements. As the separationmethod, a method is used, in which the thin film compound solar cell isfixed to a stage by vacuum suction, or the like, and in which theopening section formed by the mesa-etching is cut by a scriber so thatthe plurality of solar cell elements are formed.

Finally, a metallic ribbon made of Ag, or the like, for establishingelectrical connection between the respective elements, is connected bywelding, or the like, to an electrode pad on the electrode of each ofthe solar cell elements.

In the thin film compound solar cell manufactured by the manufacturingmethod as described above, the reinforcing material 9 is directly bondedto the rear surface film 8 as the base material without using anadhesive, such as wax. Thereby, a solvent for separating the reinforcingmaterial 9 need not be used, so that it is possible to prevent theproblem that the rear surface film 8 is separated by the solvent. Therear surface film 8 has heat resistance capable of withstanding atemperature higher than the annealing temperature of the surfaceelectrode 13, and hence it is possible to anneal the surface electrode13 after the rear surface film S is formed on the rear surface electrode7.

When the surface electrode 13 and the rear surface electrode 7 areannealed, the adhesive property of the electrodes with the cell mainbody is improved, so as to prevent each of the electrodes from beingseparated. Further, the contact resistance between the electrodes andthe compound semiconductor layer can be reduced, so that the conversionefficiency is increased.

Further, the structure, in which the surface electrode 13 is not coveredwith a surface film, is adopted, and hence the surface electrode 13 isexposed. Thus, after the substrate 1 is removed, the metallic ribbon canbe connected, and hence it is not necessary that, as in the conventionalcase, the metallic ribbon is protected when the substrate is removed. Asa result, the number of processes can be reduced.

Further, a highly heat-resistant film, such as polyimide, is used as thebase material, and thereby the film itself serves as a supporting body.Therefore, even when an external force is applied, the photovoltaic cellis prevented from being broken. Further, since the amount of warpage ofthe photovoltaic cell is changed according to the thickness of the film,the warpage of the cell can be reduced in such a manner that, when thefilm is formed, the thickness of the film is adjusted over the wholecell according to the thickness of the cell.

Note that the present invention is not limited to the above describedembodiment, but numerous modifications and changes can be obviously madetherein without departing from the spirit and scope of the presentinvention. In another method for manufacturing a thin film compoundsolar cell, after process 4 described above, the substrate 1 isseparated from the cell main body, and the etching-stop layer 2 isremoved. Then, the cell main body is mesa-etched as shown in FIG. 21. Aprotective film made of a photoresist is formed on the contact layer 3,and the patterning for forming the surface electrode is performed. Anelectrode material is laminated on the protective film and on thecontact layer 3 in the opening section. When the protective film isremoved, the surface electrode 13 is formed as shown in FIG. 22.

Thereafter, as shown in FIG. 23, the contact layer 3 is etched by thecontact layer etching. At this time, the surface electrode 13 is used asan etching mask. Subsequently, the reinforcing material 9 is separatedfrom the cell main body, and the surface electrode 13 is annealed. Then,the thin film compound solar cell is separated into a plurality of solarcell elements, and the metallic ribbon is finally connected to each ofthe solar cell elements.

With this manufacturing method, the surface electrode 13 serves as aprotective film in the contact layer etching. Thus, as compared with theabove-described manufacturing method, the process of forming theprotective film used to perform the contact layer etching can beeliminated, and hence the number of processes can be reduced.

Further, in another method for manufacturing the rear surface film whichis the base material, the rear surface film is formed by applying andsintering a solution of polyamic acid which is a polyimide precursor.That is, a solution of varnish-like polyamic acid is applied on the rearsurface electrode, and is annealed stepwise so that a polyimide film isformed.

Specifically, the polyamic acid solution is applied on the rear surfaceelectrode by a spin-coating method, or the like. Thereafter, whenannealing is first performed at 120° C. for one hour, the solvent ofpolyamic acid is evaporated, and the solution is temporarily cured.Then, the annealing temperature is increased stepwise. Finally, theannealing temperature is increased to the temperature at which thepolyamic acid is polymerized and changed to a polyimide film. By thismain curing, the polyimide film is formed.

The reason for performing the annealing stepwise in this way is that,when the annealing is performed from the beginning at a temperature atwhich the polyamic acid starts to be polymerized, the surface layer ofthe polyamic acid solution is cured prior to the curing in the inside ofthe polyamic acid solution, with the result that an air bubble includedin the solution at the time of annealing is left in the inside of thepolyamic acid solution, and that the air bubble left in the inside ofthe polyamic acid solution is expanded to form a portion at which therear surface electrode is not in close contact with the polyimide film.In addition, when the polyamic acid solution is rapidly cured, thecontraction degree of the surface layer portion of the formed polyimidefilm becomes larger than the contraction degree in the inside of thepolyimide film, so that the amount of warpage of the polyimide film isincreased and thereby the photovoltaic cell is greatly warped. Thus,when the annealing temperature is increased stepwise as described above,the air bubble is prevented from being generated in the inside of thepolyamic acid solution and the difference in the contraction degreebetween the surface layer portion and the inside of the polyimide filmcan also be prevented from occurring.

1-11. (canceled)
 12. A method for manufacturing a thin film compoundsolar cell having a cell main body in which at least one PN junction isformed by a plurality of compound semiconductor layers, each having adifferent chemical composition, the method comprising: a process offorming an etching-stop layer for suppressing infiltration of an etchingsolution from the side of a substrate, a contact layer, an emitter layermade of a first conductivity type compound semiconductor, a base layerforming a PN junction with the emitter layer, and a buffer layer; aprocess of forming a rear surface electrode on the surface on thecompound semiconductor layer; a process of annealing the rear surfaceelectrode; a process of forming a base material made of a polyimide filmby applying and annealing polyimide on the rear surface electrode; aprocess of attaching a reinforcing material on the base material; aprocess of separating and removing the substrate from the cell mainbody; a process of forming a surface electrode; a process of removingthe etching stop layer exposed by separating and removing, a process ofetching the contact layer in a predetermined pattern, a process ofmesa-etching, a process of forming a surface electrode on themesa-etched contact layer; a process of separating the reinforcingmaterial; and a process of annealing the surface electrode.
 13. A methodfor manufacturing a thin film compound solar cell having a cell mainbody in which at least one PN junction is formed by a plurality ofcompound semiconductor layers, each having a different chemicalcomposition, the method comprising: a process of forming an etching-stoplayer for suppressing infiltration of an etching solution from the sideof a substrate, a contact layer, an emitter layer made of a firstconductivity type compound semiconductor, a base layer forming a PNjunction with the emitter layer, and a buffer layer; a process offorming a rear surface electrode on the surface on the compoundsemiconductor layer; a process of annealing the rear surface electrode;a process of forming a base material made of a polyimide film byapplying and annealing polyimide on the rear surface electrode; aprocess of attaching a reinforcing material on the base material; aprocess of separating and removing the substrate from the cell mainbody; a process of forming a surface electrode; a process of removingthe etching stop layer exposed by separating and removing, a process ofetching the contact layer in a predetermined pattern, a process ofmesa-etching, a process of, after forming the surface electrode on thecontact layer, etching the contact layer using the surface electrode asan etching mask, a process of separating the reinforcing material; and aprocess of annealing the surface electrode.
 14. The method formanufacturing the thin film compound solar cell according to claim 12,wherein the polyimide film is formed by applying and sintering asolution of polyamic acid which is a polyimide precursor.
 15. The methodfor manufacturing the thin film compound solar cell according to claim12, wherein the thickness of the polyimide film is set to 15 μm or less.16. A thin film compound solar cell comprising: a compound semiconductorlayer in which at least one PN junction is formed; a surface electrodeformed on one surface of the compound semiconductor layer; a polyimidefilm formed on the other surface of the compound semiconductor layer;and a rear surface electrode sandwiched between the compoundsemiconductor layer and the polyimide film.
 17. The thin film compoundsolar cell according to claim 16, wherein the compound semiconductorlayer is made of an epitaxially grown single crystal thin film.